This invention relates to a gas sensor having a gas sensitive element which is disposed in a housing and provides an electrical output signal indicative of the existence of a specific gas or the concentration of that gas in an environmental gas atmosphere, and more particularly to a hermetic and electrically insulating seal in the housing of the gas sensor.
There are various gas sensors using a semiconductive ceramic or a solid electrolyte ceramic as the principal material of the gas sensitive element. In such gas sensors usually the gas sensitive element is constructed on a ceramic substrate by a thin-film technique or a thick-film technique, and thin lead wires are connected to the gas sensitive element and fixed to the substrate. The lead wires protrude from the substrate at one end of the substrate. Usually the substrate is partly inserted in a tubular housing, and in the housing the lead wires protruding from the substrate are respectively connected to external leads or electrode pins. The substrate is held spaced from the inner wall of the housing by a suitable fixture, and the remaining space in the housing is filled with a sealant which provides an airtight and electrically insulating seal column. Consequently, an end portion of the ceramic substrate, lead wires protruding from the substrate and end portions of the external leads or pins are buried in the seal column.
Regarding the aforementioned seal column, JP-A 60-211345 recommends to use a glass, and particularly a lead borate glass, as the sealant. The seal column of a lead borate glass is good in airtightness and electrical insulation.
However, when the lead borate glass seal is employed restrictions are placed on the use of the gas sensor at high temperatures because the glass is relatively low in melting temperature and hence insuffient in insulation resistance at high temperatures. A lead borate glass contains at least 50 wt % of PbO and has a melting temperature ranging from about 500.degree. C. to about 650.degree. C. Therefore, the gas sensor must be used on condition that the temperature of the glass seal column in the sensor does not exceed about 400.degree. C.
For many purposes it is desired to use gas sensors at considerably high temperatures, even at temperatures above 500.degree. C. A good example is the use of an oxygen sensor in the exhaust system of an automotive internal combustion engine to provide a feedback signal indicative of the concentration of oxygen in the exhaust gas for controlling the air/fuel ratio of a combustible mixture to be fed to the engine. To enhance the precision of the air/fuel ratio control it is desirable to position the oxygen sensor at a very short distance from a combustion chamber. Then the temperature of the aforementioned seal column in the oxygen sensor will become higher than 500.degree. C., and in the case of a lead borate glass seal the glass will soften and hence will become considerably low in insulation resistance. The lowering of the insulation resistance results in a leak of current between the lead wires buried in the glass seal column, and the leak of current is obstructive to accurate detection of the true output of the oxygen sensor, and hence the desire of enhancing the precision of the air/fuel ratio control cannot be met.
Besides, attention should be paid to the thermal expansion coefficient of the aforementioned seal column. For example, a lead borate glass has a linear expansion coefficient of about 7.times.10.sup.-6 /.degree.C., whereas a stainless steel which is often used as the material of the aforementioned housing has a linear expansion coefficient of about 11.times.10.sup.-6 /.degree.C. If there is such a difference in thermal expansion between the glass seal and the housing in direct contact with the glass seal, considerable thermal stresses are induced in the glass seal while the gas sensor is repeatedly heated and cooled in practical operations, and consequently cracks will appear in the glass seal column. Cracks in the seal column is unfavorable for insulation resistance and, besides, causes a leak of the sample gas such as the exhaust gas of a combustion engine through the gas sensor.